Applied Measurment Tecniques

Image intensifier

The large leftside screen detects the Xrays radiation, which is converted into electrons, driven to therightside small screen. Electron impinging such phosphor screen generates light photons. The insideof the tube is a conic shape because we are working with a large incoming radiation, which generateselectrons that need to be pointed to the small screen.

We have three part:

• Input phosphor (or first tube): It is made by a thin layer of phosphor material, transformingXray radiation in light photons. Each XRay photon can produce on average 1000 light photon.
• Photocathode (or second tube): this layer is bonded with the first tube and is usually madeof cesium and antimony. It transduces light photons into electrons, and if there is a highvoltage between the photocathode and the anode, the electrons will be accelerated towardsthe anode. The higher is the voltage drop between them, the higher will be the energyassociated to the impinging electrode on

27 X-RAY IMAGING

Thus, the purpose of X-ray fluoroscopy is to dynamic imaging based on X-ray, but most importantly, to reduce the dose in those dead times in which the operators have to adjust the reciprocal position between the patient and the cassette to obtain an optimal image.

To sum up, the Image Intensifier provides an output greater than the input because of the amplification factor:

• Geometric factor: All the electrons are converged into a small area instead of being distributed in a wide one. The light is output very intense and in a very little beam and optical systems (lenses) can zoom the image, so there is no issue linked to eye capabilities of perceiving the image.
• Energy factor: the voltage drop between cathode and anode: combining these two factors, the gain is 10000 to 30000.

As it is possible to see in the scheme below, the Image Intensifier is embedded in a Measurement chain. This chain is a feedback system. The output phosphor is used to

sensitivity of the camera, depending on the intensity of the impinging light; typically I keep my camera iris as large as possible, so less intensity is required (less dose).

Regarding the power supply, the image intensifier requires at least five voltage control for the three stages of this device. Three of them are regulating the plates likewise in order to focus the electrons to the output phosphor (E1,E2,E3) and the other two regulate the voltage drop between the photocathode (P) and the anode (A), that need to be a very high value to accelerate the electrons towards the anode. As is shown in the picture below a specific circuit is rectifying the supply voltage to 27 V from the AC power line, and then a regulation block outputs all the voltages needed in the image intensifier.

Modalities of operation

1. Manual mode: the operator is regulating the voltage and current manually. Is always available, is not the best to be featured and it is recommended only for expert

1) Continuous Fluoroscopy mode (CF): is the mode in which the X-ray tube emits a continuous beam of radiation. This mode is used for procedures that require real-time imaging, such as guiding catheters or needles.

2) Automatic Emission Control mode (AEC): is the mode with a feedback loop that controls the amount of radiation reaching the detector. The system keeps the radiation low while maintaining acceptable image quality. The control is exerted quickly.

3) Pulse digital mode (PDM): modifies the fluoroscopic output by cutting out exposure between pulses. The pulse frequency should be as low as possible while still achieving the desired results. For most non-cardiac procedures, a pulse rate of 10 pulses/s or less is sufficient. PDM substantially reduces the exposure rate.

Fluoroscopy units:

There is no standard recommendation for the location of the X-ray tube and the image intensifier. However, the tube is usually positioned above the patient and the image intensifier below, rigidly fixed on a frame. The table can typically slide for positioning.

• Coronary tree: introduction of a contrast agent, if a part of the tissue is not visualized that means that the vessel is occluded (cause of stroke).
• Mammography: exploration of breasts. A very low dose of X-ray but very focalized. The target area is well confined. The volume is always reduced, and the thickness is not an issue, so the system is optimized to that kind of exploration.

1. Direct method (static: X-rays, exposure and cassette system)
2. Direct method (dynamic: X-rays, exposure,

Intensification of image and TV system - Direct method - It mounts the light distributor system, which allows to store information into an HD and run in real-time. This layout allows to run the PDM modality. At more than 30 frames per second, it is more or less real-time for our eyes. In that way, the scopy can be run for long times (tens of minutes to some hours); this becomes mandatory for surgery applications of X-ray imaging.

The typical frame rate for fluoroscopy is 30 fps, which is low for our eyes (we won't see it in real-time but as a rapid succession of images), but enough to investigate physiological processes. The lower the frame rate, the lower the dose delivered to the patient because the system turns on fewer times during a second.

The dose rate is regulated:

1. AEC + ionic camera;
2. ADR + measure of bright intensity in the I.I. with a photodiode;
3. ADR from video signal in the TV System.

Advantages of this modality:

• Thermal load to the anode: the anode load is interrupted.